1. Field of the Invention
This invention relates to conductors for solid state devices and more particularly to multiple layer conductors.
2. Description of Prior Art
To increase performance of integrated circuits, the trend in the semiconductor industry is toward smaller devices electrically connected by very narrow thin film conductors. In current products, typical current densities to which these thin film conductors are subjected are in the mid-10.sup.5 amperes/cm.sup.2 range. New device designs call for higher current densities, in some cases, as high as the mid-10.sup.6 amperes/cm.sup.2 range.
Direct current in these current density ranges induces motion of the atoms comprising the thin film conductor, an effect known as electromigration. Electromigration induces crack or void formation in the conductor which, over a period of time, can result in conductor failure. The rate of electromigration is dependent on the current density imposed on the conductor, the conductor temperature, and the properties of the conductor material. In high current density applications, potential conductor failure due to electromigration can severely limit the reliability of the circuit. Electromigration can cause an additional problem in semiconductor devices when a passivation layer such as glass, silicon nitride, or silicon dioxide is overlaid on the device, as is typically done in the industry. This layer can fracture as a result of the removal and build-up of conductor material. This fracture can expose some of the devices or conductors to atmospheric corrosion.
Conventionally in the microelectronics industry, aluminum (Al) has been used as the conductor material. For the current density requirements of some Large Scale Integration (LSI) semiconductor applications, pure aluminum conductors are unreliable because of their susceptibility to electromigration. Several metallurgical systems have been proposed previously in which electromigration occurs more slowly than in pure alumina conductors, thus leading to longer conductor life or the ability to impose higher current densities on the conductor. Among the proposed metallurgical systems are Al-Cu, Ta/Au/Ta, Al-intermetallic configurations and Au-intermetallic configurations.
The basic requirements which a metal or metal system (i.e., a configuration of one or more metals, either as an alloy, pseudo-alloy, or layered structure) must achieve in order to be a suitable candidate for use as a thin film conductor in integrated, solid state circuits are summarized as follows:
(1) high electrical conductivity--only materials having high electrical conductivity are considered because low conductivity values lead to excessive Joule heating and too large a voltage loss in the conductor, or the alternative of making the conductor with an unacceptably large cross-sectional area.
(2) corrosion resistance--conductor materials which are susceptible to corrosion, even if they are relatively resistant to electromigration induced failure, are prone to fail from corrosion effects over the intended lifetime of the device.
(3) chemical stability with regard to other materials with which the conductor will be in contact in the integrated circuit; such materials are typically silicon, silicon dioxide, silicon nitride, or gadolinium gallium garnet.
(4) adhesion to the substrate on which the thin film conductor is fabricated, and adhesion to the passivating layer applied over the thin film conductor. This passivating layer insulates the conductor and provides a degree of protection against corrosion. Thus, adhesion to such materials as silicon, silicon dioxide and silicon nitride is required.
(5) compatibility with later processing of the integrated circuit. This compatibility may take the form of metallurgical stability under elevated temperature conditions encountered in processing, and resistance to chemical attack by certain chemical agents to which the conductor may become exposed in processing.
(6) ability to be deposited by the fabrication techniques in common use within the industry for metal deposition, for example by vapor deposition or by sputter deposition.
(7) certain properties of the film's grain structure are also important. In order to obtain adequate lithographic linewidth resolution, the film should be small grained, with a grain size not exceeding about one-third of the required linewidth. Uniformity of grain size and preferred crystallographic orientation of the grains are also factors which promote longer electromigration limited conductor lifetimes. Fine grained films are also smoother, which is a desirable quality in semiconductor applications to lessen difficulties associated with covering the conductor with an overlayer.
No single metal is exceptionally well suited to satisfying all of the above requirements. Aluminum, copper, silver, and gold provide high conductivity, but of them only Al adheres well to silicon, silicon dioxide, and silicon nitride. However, the direct contact of Al to silicon must be avoided in a number of semiconductor applications because of mutual solubility, which results in p-doping of silicon and the possibility of junction penetration. Also, pure Al is highly susceptible to electromigration.
Using metal systems (i.e., combinations of metals), significant progress has been achieved toward reducing the effects of electromigration in thin film conductors. A recent review of advances in using Al-based metal systems is provided in U.S. Pat. No. 4,017,890 of Howard et al which is commonly assigned.
Adhesion problems encountered in using Ag, Au or Cu conductors have been overcome by using adhesion layers between the conductor and the material to which it must adhere. However, Ag is susceptible to corrosion. Cu is also susceptible to corrosion, although to a lesser degree.
Au provides a high degree of resistance to electromigration induced failure, and to corrosion, is not attacked by most chemical agents, and is easily deposited in pure form by vapor deposition, sputter deposition or electroplating. With proper techniques of deposition, it forms a small, uniform grain structure, with strong preferred orientation of the (111) crystallographic direction normal to the film plane. Au in conjunction with Ta adhesion layers has been proposed as an electromigration resistant metallurgy for semiconductor applications by Riseman et al in U.S. Pat. No. 3,617,816 which is also commonly assigned. Au in conjunction with adhesion layers of Nb, Hf, or Zr, which form intermetallic compounds with Au, has been proposed by Gangulee et al in U.S. Pat. No. 4,166,279, which is commonly assigned, wherein there is also a review of other advances in using Au-based metal systems.